I've seen cases and other lenses that can attach to the phone that add effects like fish eye and wide angle. These don't look as easy to use as the case system or even the magnetic system but darn it if they aren't cheap. A Jackson will get you about 4 lenses. I'm going for the fish eye, wide angle, polarized, and star burst lenses.
According the review on iPhoneography, they're OK. Couple that review with the actual cost and I can justify the $20. Browse over to the review and check out his sample shots.